Field of the Invention
This invention relates to light emitters and, more particularly, to light emitter packages with components arranged to withstand thermal stresses.
Description of the Related Art
Light emitters are an important class of solid-state devices that convert electrical energy to light. One such light emitter is a light emitting diode (LED) which generally includes an active region of semi-conductive material sandwiched between two oppositely doped regions. When a bias is applied across the doped regions, holes and electrons are injected into the active region where they recombine to generate light. The light can be emitted from the active region and through the surfaces of the LED.
LEDs are generally divided into classes depending on their power rating. Although there is no standard range for the different classes, low power LEDs typically have a power rating in the range of 0.1 Watts to 0.3 Watts, or lower, and high power LEDs typically have a rating in the range of 0.5 Watts to 1.0 Watt, or higher.
Conventional packaging for low power LEDs typically includes a reflector cup with the LED mounted at the bottom of the cup. Cathode and anode leads are electrically coupled to the LED to provide power. The cathode lead can extend through the reflector cup and the anode lead can be wire bonded. The main function of the reflector cup is to redirect light emitted in certain directions in order to control the far-field intensity pattern of the LED. The reflector cup can include a highly reflective surface finish and can be plate stamped or metal plated with a metal such as aluminum (Al) or silver (Ag).
The entire structure can be encased in a transparent, hard encapsulant such as a plastic or epoxy. The encapsulant serves a number of functions. One function is to provide a hermetic seal for the LED chip. In another function, light refracts at the encapsulant/air interface, so that the outside shape of the encapsulant can act as a lens to further control the intensity pattern of the LED.
One disadvantage of this packaging arrangement, however, is that the LED chip, the reflector cup, and the encapsulant each generally have different coefficients of thermal expansion (CTE). Hence, during operational heating cycles they expand and contract at different rates, which can place a high mechanical stress on the device. In particular, epoxies and silicones typically used for the encapsulant have a CTE that is very different from the CTE of metals or ceramics. The CTE mismatch can also be exacerbated by constraints imposed by the manufacturing flow, such as during epoxy curing. In addition, these packages do not dissipate heat from the LED chip efficiently as they lack good thermal properties. However, because the LED operates at low power, the amount of heat it produces is relatively low so that the differences in CTE do not result in unacceptable failure rates.
High power LEDs, however, are generally larger, use larger packaging components, and generate higher amounts of heat. As a result, the CTE mismatch has a much larger impact on reliability and if the low-power LED type packaging is used, the differences in CTE for the packaging components can result in unacceptable failure rates. One of the most common failures is fracturing or cracking of the encapsulant.
High power LED packages have been introduced having a heat spreader that serves as a rigid platform for the remainder of the components, and is made of a material with high thermal conductivity such as a metal or ceramic that helps to radiate heat away from the LED chip. A reflector cup is mounted to the platform with the LED chip mounted at the bottom of the cup. The LED chip is contacted by wire bonds from the rigid platform. The reflector cup, LED chip and wire bonds are encased in an optically clear material that provides environmental protection. To compensate for the different coefficients of thermal expansion (CTE) of the package components, the optically clear material can include a soft gel such as silicone. As the different components expand and contract through thermal cycles, the soft gel readily deforms and compensates for the different CTEs.
However, soft gel is not as robust as plastics, epoxies, and glass, and cannot be used in some harsh environments without a coating or cover to act as a hermetic seal, which adds complexity to the LED fabrication process. The soft gel also tends to absorb water, which can shorten the LED's lifespan. It is also more difficult to shape soft gels to control the emission pattern of the LED package.
Other high power LED packages have been introduced that utilize a hard epoxy encapsulant, with one such device not utilizing a reflector cup inside the encapsulant. Instead, a second region is included on the heat spreader, with a section of the second region stamped, molded or etched to form a depression that can be coated with a reflective material. The LED chip is then placed at the base of the depression and is contacted. A hard epoxy or silicone fills the depression, covering the LED and any wire bonds. This arrangement reduces, but does not eliminate, the fractures and cracking of the epoxy or silicone encapsulant. This arrangement can also suffer from a different problem of the epoxy or silicone encapsulant delaminating and peeling away from the surfaces of the depression through the LED's thermal cycles.
U.S. Pat. No. 6,274,924 to Carey et al. discloses another high power LED package that includes a heat sinking slug that is inserted into an insert molded leadframe. The slug can include a reflector cup with the LED chip and thermally conductive submount arranged at the base of the cup. Metal leads are electrically and thermally isolated from the slug. An optical lens is added by mounting a thermoplastic lens over the slug. The lens can be molded to leave room for a soft encapsulant between the LED and the inside surface of the lens. This invention claims to operate reliably under high power conditions, but is complex, difficult to manufacture, and expensive. The thermoplastic lens also does not survive high temperatures typically used for the process of soldering LEDs to a printed circuit board.